The present invention relates to a wire sawing device comprising wire guide cylinders supporting a wire held in position by grooves provided on the surface of the wire guide cylinders which define the interval between the wires of at least one layer of wires, the wire being adapted to move with alternating or continuous movement bearing against at least one piece to be sawed.
During cutting of thin slices from a piece to be sawed, the stretched wire is both guided and tensioned by the wire guide cylinders. These wire guide cylinders, generally coated with a layer of synthetic material, are engraved with grooves whose geometry and dimensions must be very precise. The wire is wound spirally about the wire guide cylinders and forms between two wire guide cylinders at least one layer of parallel wires whose successive inter-wire distance fixes the thickness of the slices. Moreover, the plane of the layer of wires forms, in known devices, a fixed angle generally perpendicular to the direction of sawing, which can lead to undulations and striations on the surface of the slices in the case of general lateral movement of the layer of wires resulting from thermal oscillations for example. These undulations, even of several micrometers of amplitude, suffice to render the slices unusable for certain applications such as silicon for the semiconductor industry. The thermal variations arise from two principal causes, namely the energy used for sawing properly so called, and the energy dissipated in the mechanical elements such as the roller bearings and particularly in the bearings supporting the wire guide cylinders. However, the requirement of the industry to provide more and more first class machines, whether as to productivity or as to the dimensions of the pieces to be sawed, requires higher and higher powers for sawing properly so called and mechanical elements that are progressively greater in size, hence which dissipate more energy. If therefore it is not possible to eliminate the dissipation of energy in the entire machine, the cutting energy must be controlled and recovered by a cooling system. On the other hand, the energy generated in the mechanical elements is not often controlled and recovered other than in an indirect manner because of the mechanical complexity of the more sensitive elements. One of the causes of heating of the support bearings of the wire guide cylinders is the great load on these due to the tension of the layer of wires. The latter can amount to up to several tons, for large capacity sawing devices. From the heating that thus arises, there result thermal variations and thermal oscillations generating undulations or waves in the sawed slices.
Wire sawing devices of the mentioned type with oscillation of the layer of wires or of the piece to be sawed are already known, especially in the industry of electronic components of ferrites, quartz and silica, to obtain thin slices of material such as poly or monocrystalline silicon or new materials such as GaAs, InP, GGG or else quartz, synthetic sapphire, ceramic materials. The high price of these materials renders wire sawing more attractive in comparison to other techniques such as diamond disc sawing. But all the devices provided have wire guide cylinders whose geometric arrangement gives rise to a heavy load on the bearings and because of this gives rise to a great dissipation of energy and hence poor control of temperature and as a result altered precision.
The present invention has for its object to overcome these principal drawbacks, and the invention is characterized to this end by the fact that the sawing device comprises two external wire guide cylinders and two internal wire guide cylinders disposed between the external wire guide cylinders, whose diameter is smaller than that of the interior wire guide cylinders, and by the fact that these external wire guide cylinders are driven in rotation by drive members to cause the movement of the wires of the layer of wires, whilst the internal wire guide cylinders ensure a guide function without driving the wires of the layer of wires, the wire being spirally wound about the assembly of wire guide cylinders without the layers crossing.
The combination of these characteristics permits overcoming the mentioned drawbacks by separating the function of driving the layer of wires from the function of guiding said layer of wires. This separation of the functions comes about by placing in parallel and on a same plane, four wire guide cylinders. The two end cylinders serve for driving and hence are accordingly motor driven with a high power, and the two internal cylinders serve for guiding the layer of wires. These two internal wire guide cylinders are those which give precision to the slices. As they are subjected only to very low loads, their bearings heat up very little and because of this have almost no influence on the local thermal equilibrium. As a result, there is higher precision of the pieces to be sawed. During sawing with a layer of non-crossing parallel wires, it will be necessary that the external wire guide cylinders or drive cylinders have a smaller diameter than the internal cylinders so as to give to the layer of wires a sufficient guide surface to ensure precision of guiding.
Naturally, these internal wire guide cylinders can be power driven so as to compensate the effects of inertia due to the weight of the wire guide cylinders themselves, especially during back and forth sawing. During continuous sawing, the friction forces of the layer on the internal wire guide cylinders can be sufficient to drive them without a motor. In the case of motorized drive of the internal wire guide cylinders, this motorized drive of low power serves only to drive without slipping the wire guide cylinders and in no case will serve to drive the layer of wires for sawing. The separation of the drive and guide functions must necessarily be provided. It will also be necessary that the grooves engraved in the wire guide cylinders have a high precision, resulting in the quality of the slices.
The surface condition of the obtained pieces, if it is not controlled, can throw doubt on the whole process. This sawing technique thus further requires a minimization of the energy dissipated in the sawing region so as to avoid any thermal movement due to the expansion resulting from energy dissipated in sensitive regions such as the bearings of the wire guide cylinders. The separation of the drive and guiding functions, as proposed by the invention, permits minimizing these defects in the course of sawing. Poor thermal control, even at a low level, will result in slices that are unacceptable for subsequent processes.
The requirements of electronic applications, for example connected to the increasing dimensions of the pieces to be sawed, require that even the smallest variations be avoided. It is accordingly therefore no longer sufficient to saw slices without eliminating forces arising from the wire guide cylinders, and hence without separating the guide and drive functions of the layer of wires.
The sawing device with separation of the guiding and drive functions of the wire guide, which is the object of the present invention, permits minimizing the thermal variations in the sawing region, hence producing pieces of increased precision.
The geometric arrangement of the wire guide permitting the separation of the sawing forces from the guide forces, according to the present invention, comprises an assembly of four wire guide cylinders disposed parallel in a same plane, the external wire guide cylinders imparting the sawing force having a smaller diameter than the internal cylinders effecting guiding. Moreover, during reciprocating operation, the internal wire guide cylinders can also be power driven so as to avoid slipping which could result due to inertia of these latter. In this case, there will be no slipping during acceleration and deceleration between the wire of the layer of wires and the surface of the guiding wire guide cylinders. The use of this type of device permits producing pieces of increased precision.